Moving a message replaces every mailbox it sits in; adding a label leaves the others alone — and both are exposed separately, singly and in bulk, which is the difference between filing mail correctly and flattening it. Two things to know before you plan around it: the metadata-only variants exist so a search does not drag message bodies into context, and Fastmail's API tokens do not cover calendars, so those tools go over CalDAV with their own app password.
An unofficial Fastmail server built on JMAP for mail and contacts, with calendar handled over CalDAV: 52 tools covering search, sending, bulk filing, attachments, contacts and events.
- Mailboxes listed with their unread and total counts, looked up by full path, created, and summarised account-wide
- Mail searched by content or by several criteria at once — sender, recipient, subject, date range, attachment, unread state
- Messages sent, replied to with the threading headers built for you, or saved as drafts
- Read state, deletion and moves applied to one message or a batch, with a dry-run mode for trying a bulk operation before it happens
- Labels added and removed without disturbing the mailboxes a message already sits in
- A whole conversation thread pulled in one call, and metadata-only variants of the list, search and thread tools when the bodies are not wanted in context
- Attachments listed and downloaded into a directory you choose, or saved straight to WebDAV storage
- Contacts listed, fetched, searched, and created or updated where the token allows it
- Calendars and events listed, an event created or patched without losing the fields you did not touch, and an event deleted
- A readiness check that reports which of these your account and credentials actually allow
Node 20 or newer and a Fastmail API token, created under Settings → Privacy & Security; contact writes need a token with read-write contacts scope. Calendar is separate: it runs over CalDAV and needs an app-specific password alongside your address, and WebDAV saving needs its own URL and credentials. Attachment downloads are confined to the directory you configure. It installs from a clone, or as a packaged desktop extension built from the repository. This is a community project, not affiliated with Fastmail.
Build from source — clone the repository and build it, then point your client at the binary